RFR: 8357681: Fixed the DigitList::toString method causing incorrect results during debugging

[Roger Riggs]

On Tue, 27 May 2025 19:44:55 GMT, Shaojin Wen <swen at openjdk.org> wrote:

> To test this concurrency problem, we need to build a concurrent competition test, which is not a simple task, so I did not add it.

The label `noreg-hard` would be appropriate to indicate a test was not created due to difficulty.
That label though conflicts with the current label `noreg-trivial` implying there is an existing test and the change is not significant. It will look odd, but both labels make sense.
[OpenJDK Developers guide](https://openjdk.org/guide/#jbs-label-dictionary)

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PR Comment: https://git.openjdk.org/jdk/pull/25288#issuecomment-2913885924